Conventionally, in a DC power-supply device that uses, as a load, an inverter that drives a compressor motor or the like used in an air conditioner, a heat-pump water heater, a refrigerator, a freezer, and the like, as a configuration that converts an alternating current into a direct current, for example, a configuration that converts a single-phase alternating current into a direct current (for example, Patent Literature 1) and a configuration that converts a three-phase alternating current into a direct current have been disclosed (for example, Patent Literature 2). In these conventional techniques, switching loss can be decreased by keeping the switching frequency low, thereby enabling to achieve high efficiency.
In the conventional technique described above, by serially connecting a plurality of capacitors between both terminals of the load and charging the capacitors, a voltage value to be supplied to the load is controlled. In such a configuration, in order to stabilize a supply voltage to the load and achieve high reliability and long life of the capacitors, the voltage of the capacitors needs to be balanced. As such a technique, for example, there have been disclosed a configuration in which in a half-bridge AC/DC converter, an unbalance detection circuit including first and second detection resistors having the same resistance value is provided between both terminals of the load, to detect a middle-point voltage as an unbalance detection voltage, thereby executing control such that the unbalance detection voltage becomes constant (for example, Patent Literature 3), for example, a configuration in which in a DC/DC converter, a voltage detector that detects a voltage between both ends of two capacitors is provided, to output a pulse-modulated pulse on the basis of an output of the voltage detector, thereby to appropriately control a switching operation (for example, Patent Literature 4).